Spray Model Based Phenomenological Combustion Description and Experimental Validation for a Dual Fuel Engine 2017-24-0098

The operation of dual fuel engines, operated with natural gas as main fuel, offers the potential of substantial savings in CO2. Nevertheless, the operating map area where low pollutant emissions are produced is very narrow. Especially at low load, the raw exhaust gas contains high concentrations of unburned methane and, with high pilot fuel portions due to ignition limitations, also soot. The analysis of the combustion in those conditions in particular is not trivial, since multiple combustion modes are present concurrently.

The present work focuses on the evaluation of the individual combustion modes of a dual fuel engine, operated with natural gas as main and diesel as pilot fuel, using a combustion model. The combustion has been split in two partwise concurrent combustion phases: the auto-ignition phase and the premixed flame propagation phase. In order to calculate the amount of fuel and equivalence ratio distribution of the diesel and substrate at ignition, a simplified spray model is used. The trapped natural gas within the spray volume provides the basis for the distribution of fuel burnt in the auto-ignition phase and in the premixed flame propagation. The sum of the auto-ignition combustion mode and the premixed flame propagation combustion mode represents the total of the heat released, disregarding the cool flame, heat losses and unburned fuel.

The description has been calibrated on a 4 cylinder, 2 litre Volkswagen Industrial Diesel Engine, modified to include natural gas port injection. The engine was operated with a wide variation of settings, ranging from low to high load, globally lean and stoichiometric with pilot fuel mass ratio from 2% to 50% and different EGR rates. The heat release rate derived from the pressure curve shows very good agreement with the sum of the individual combustion modes in all the abovementioned engine operating conditions. The ability to distinguish between the individual combustion modes, offered by the developed combustion description herein aids in the understanding of the engine operation limitations. The predictive model shows a strong dependency on the ignition delay which is itself challenging to model. Consequently the accuracy of predicted combustion characteristics are limited to the accuracy of the modelled ignition delay.